Reject invalid server names (#3480)

Make sure that server_names used in auth headers are sane, and reject them with
a sensible error code, before they disappear off into the depths of the system.

synapse/http/endpoint.py

@@ -38,6 +38,36 @@ _Server = collections.namedtuple(
 )
 
 
+def parse_server_name(server_name):
+    """Split a server name into host/port parts.
+
+    Does some basic sanity checking of the
+
+    Args:
+        server_name (str): server name to parse
+
+    Returns:
+        Tuple[str, int|None]: host/port parts.
+
+    Raises:
+        ValueError if the server name could not be parsed.
+    """
+    try:
+        if server_name[-1] == ']':
+            # ipv6 literal, hopefully
+            if server_name[0] != '[':
+                raise Exception()
+
+            return server_name, None
+
+        domain_port = server_name.rsplit(":", 1)
+        domain = domain_port[0]
+        port = int(domain_port[1]) if domain_port[1:] else None
+        return domain, port
+    except Exception:
+        raise ValueError("Invalid server name '%s'" % server_name)
+
+
 def matrix_federation_endpoint(reactor, destination, ssl_context_factory=None,
                                timeout=None):
     """Construct an endpoint for the given matrix destination.
@@ -50,9 +80,7 @@ def matrix_federation_endpoint(reactor, destination, ssl_context_factory=None,
         timeout (int): connection timeout in seconds
     """
 
-    domain_port = destination.split(":")
-    domain = domain_port[0]
-    port = int(domain_port[1]) if domain_port[1:] else None
+    domain, port = parse_server_name(destination)
 
     endpoint_kw_args = {}